This invention relates to carbon dioxide concentration sensors that determine the concentration of carbon dioxide in a given environment while carrying out calibration based on the carbon dioxide concentration in the reference atmosphere.
Outputs from carbon dioxide sensors employing solid electrolytes or infrared rays vary with time. Therefore, such sensors correct their sensitivity by using calibration data prepared by statistically processing the chronologically measured and stored carbon dioxide concentrations in the reference air, as described in Japanese Provisional Patent Publication No. 249073 of 1993.
This type of sensors permit correction of changes in the carbon dioxide sensing means with age if the concentration of carbon dioxide in the environment has lowered to the level of carbon dioxide in the reference air. In modern offices and other similar extremely airtight buildings, however, accurate calibration is practically difficult to achieve because it takes a very long time for the carbon dioxide concentration to reach the minimum value.
A carbon dioxide sensor of this invention comprises a carbon dioxide sensing unit, an arithmetic unit that predicts the minimum value of carbon dioxide concentration that is regulated by the diffusion of the atmosphere by checking if the signal from a carbon dioxide sensing unit indicates that the carbon dioxide concentration has entered the declining phase, and a calibrating unit that updates the sensitivity correction factor based on the predicted lowest carbon dioxide concentration.
When the carbon dioxide in the environment is decreasing, its concentration is regulated by diffusion equation etc. and changes substantially univocally. Thus, the sensor according to this invention can predict with accuracy the minimum value of carbon dioxide concentration. Therefore, calibration can be made without waiting until the carbon dioxide concentration actually drops to the minimum value.
The object of this invention is to provide carbon dioxide concentration sensors that permit correcting sensitivity, zero point, and other calibration factors without waiting until the carbon dioxide concentration in the environment reaches the minimum value.